In image-guided radiation therapy (IGRT), on-board X-ray imaging has evolved from two to three dimensions, and cone-beam CT (CBCT) is being increasingly utilized to improve the therapy or treatment performance. CBCT imaging can be performed just before, during, or after treatment without moving the patient and is expected to yield more faithful and relevant patient images at treatment time compared to the planning multi-detector CT (MDCT) images taken before patients start their fractionated radiotherapy regimen. In current clinical practices, CBCT is used mainly to provide positioning information for treatment setup. In such cases, quantitative CBCT images with high CT number accuracies generally are not required. With a Hounsfield unit (HU) calibration, CBCT-based dose calculation achieves a satisfactory accuracy when the scanned objects are small or largely uniform. More demanding applications, including calculation of dose distribution on a complex geometry and tumor delineation for adaptive radiation therapy, however, require high-quality CBCT images. In these cases, applications of CBCT can be hindered by large shading artifacts in the reconstructed images.
The shading artifacts in CBCT images result from several non-idealities, including scattered radiation, beam hardening effects, detector lag, non-linear detector gains and the use of a bow-tie filter. Among these, the artifacts resulting from the severe scatter contamination due to the large size of an illuminated volume are most severe. Scatter signals have dominant low-frequency components in spatial distribution. On a CBCT system, without scatter correction, the scatter-to-primary ratio (SPR) is typically about 2 on a mid-size volume and can be up to about 5 on a human torso. The large scatter signals lead to CT number errors up to about 350 HU.
To overcome these obstacles, many CBCT correction methods have been proposed. These methods can be divided into two major categories: pre-processing and post-processing methods. By way of explanation, the pre-processing methods reduce scatter artifacts by preventing the scattered photons from reaching the detector, while, the post-processing methods involve the application of correction algorithms to scatter-contaminated projection images after a conventional data acquisition. Unfortunately, despite the advancements made in correction methods, there remains a need for improved correction methods. It is to the provision of such methods that the various embodiments of the present inventions are directed.